1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly, to a method of adjusting a signal power in a variable data rate mode in a mobile communication system.
2. Discussion of the Related Art
Generally, in a 3rd generation mobile communication system of a CDMA type, a mobile station has a multi-channel structure that simultaneously transmits several traffic channels in order to provide diverse multimedia services such as voice, image, and data in addition to transmitting only one traffic channel as in the IS-95A.
Also, in a reverse link, a base station performs a coherent demodulation in order to improve the performance. The mobile station transmits a pilot channel along with a plurality of traffic channels so that the base station performs the coherent demodulation of the traffic channels.
A traffic-to-pilot power ratio is determined in accordance with a coding rate, a desired signal-to-interference ratio (SIR), a transmission rate, etc.
FIG. 1 is a block diagram explaining a conventional process of generating a multi-channel transmission signal.
As shown in FIG. 1, a mobile station passes a pilot channel and traffic channels (i.e., a dedicated control channel, a supplemental channel, and a fundamental channel) through a spreading unit 110 and a baseband filter 106 to transmit the channels to a base station.
At this time, gains of the respective traffic channels are adjusted through a relative gain section 102 before being spread so that the traffic channels are transmitted with relative transmission power ratios GF, GC, and GS of the traffic channels to the pilot channel.
Here, GF represents a value of the relative transmission power ratio of the fundamental channel for transmitting information such as voice and so on, GC represents a value of the relative transmission power ratio of the dedicated control channel used to transmit dedicated control information, and GS represents a value of the relative transmission power ratio of the supplemental channel used to transmit data.
The transmission power of the pilot channel is adjusted by a closed-loop power control through a reverse link.
The transmission power control of the traffic channels is performed in a manner that the transmission power ratio of the traffic channel to the pilot channel is maintained constantly.
The closed-loop power control is classified into two following types.
First, an inner-loop power control type measures the signal-to-interference ratio (SIR) of a received signal from a signal power of the pilot channel to compare the SIR with a power control threshold value, and transmits power control bits through a forward link.
Second, an outer-loop power control type periodically adjusts the power control threshold value in order to maintain a frame error rate (FER) desired by a radio channel that changes according to time.
At this time, the threshold value of power control to be used in a receiving end for each data rate is differently defined. In other words, the power level of a reference pilot signal to be used in a sending end for each data rate is differently defined.
Also, the power ratio of the traffic signal to the pilot signal is differently defined for the respective data rate.
Meanwhile, there have been proposed a 1x cdma2000 system designed to support a voice service of a circuit switched mode and an intermediate-speed and high-speed data service, a high data rate (HDR) or 1x-evolution data only (1x-EV DO) system proposed only for a high-speed packet data communication, and a 1x-evolution data and voice (1x-EV DV) system that is an integrated version of the two systems as described above. The data rate control in the above systems is performed as follows. Hereinafter, these systems will be generically named a 1x system.
There is a variable data rate mode among operation modes of the conventional 1x system.
Especially, in the variable data rate mode in the reverse link, a set of data rates fixed to one another should be defined between the mobile station and the base station in a negotiation stage.
Generally, in defining the data rate set, about 3 data rates are determined as a hypothesis set for the variable data rate, and the mobile station automatically changes the data rates within the set.
Specifically, if the mobile station judges the remaining power is not sufficient to maintain the present data rate as it is checking the remaining power that can be used by itself, it changes its own data rate to a lower data rate existing in the hypothesis set.
Then, the base station recognizes the changed data rate through a blind rate detection.
At this time, in order to control the signal power corresponding to the changed data rate, the 1x system uses the following methods.
As described above, the power ratio of the traffic signal to the pilot signal should be changed for the respective data rates. That is, in case of a high data rate, the level of the received pilot signal should also be heightened. As described above, if the power level of the received pilot signal is separately controlled for the respective data rate, the variable data rate operation becomes impossible. That is, since the data rate of the reverse link currently transmitted cannot be known until the base station completely receives one frame, it is impossible to determine the power control threshold value to be used in the inner loop.
In order to solve the above-described problem, there has been proposed a method of using the reference pilot power level for the maximum data rate allocated in the hypothesis set with respect to other data rates in the hypothesis set. That is, in case of using the power level of the reference pilot signal to the maximum data rate in other data rates in the hypothesis set, i.e., in case that the mobile station transmits the corresponding signal with a data rate different from the maximum data rate, the power ratio value of the supplemental channel signal to the pilot signal becomes different from the original power ratio value.
An example of this process will now be explained.
It is assumed that the hypothesis set to be used is put as {9.6 kbps, 19.2 kbps, 38.4 kbps} through the negotiation stage between the mobile station and the base station. In this case, the optimum power ratios of the pilot signal to the traffic signal are determined as follows for the respective data rates.
(Pilot:Traffic)=(1:2.37), 9.6 kbps
(Pilot:Traffic)=(1.02:4.3), 19.2 kbps
(Pilot:Traffic)=(1.37:7.7), 38.4 kbps
At this time, the power ratios of the traffic signal to the pilot signal for each data rate used in the variable data rate will be (1.37:2.37), (1.37:4.3), and (1.37:7.7) with respect to 9.6 kbps, 19.2 kbps, and 38.4 kbps, respectively.
Next, the variable data rate used in the 1x-EV DO system is driven in the following manner.
First, the mobile station has a set of 5 transmission data rates {9.6 kbps, 19.2 kbps, 38.4 kbps, 76.8 kbps, 153.6 kbps}.
The mobile station starts the transmission with the minimum data rate of 9.6 kbps. Then, for each frame transmission time point, the mobile station performs a p-persistent test, and increases the data rate by one stage only in case of passing this test.
At this time, the probability value that the mobile station increases the data rate in the set of the above-described data rates gets smaller as the data rate becomes higher.
Then, the base station measures the power level of an interference signal, and if the power level of the interference signal becomes higher than a certain threshold value during the measurement, it generates a common reverse activity (RA) bit to transmit the common RA bit to the mobile stations.
The mobile stations that received the RA bit perform the p-persistent test, and only the mobile stations determined to lower the rate during the test lower the data rate by one stage.
The mobile station should accurately transfer the data rate selected during the above process to the base station through a reverse rate indicator (RRI) channel.
In the variable rate operation process as described above, the power level of the reference pilot signal to the lowest data rate in the set of 5 data rates is also used in other data rates in the set, and the base station sets the power control threshold value to the pilot signal to be used in the inner-loop power control on the basis of the reference pilot power level.
The above-described methods, however, have the following problems if they are used in the variable data rate operation process considered in the 1x-EV DV system.
The set of variable data rates currently considered is {9.6 kbps, 19.2 kbps, 38.4 kbps, 76.8 kbps, 153.6 kbps, 307.2 kbps, 614.4 kbps, 1024 kbps} that has 8 data rates.
In this case, if the power level of the reference pilot signal to the maximum data rate of 1024 kbps is used for all the data rates that can be supported in the system, the power of the pilot signal becomes excessively large in comparison to the power of the data to be sent.
As a result, it brings the excessive increase of overhead.
Even if the power level of the reference pilot signal of 153.6 kbps that is an intermediate data rate is used, the power of the pilot signal cannot provide the power sufficient for demodulation of 8PSK that is used in 1024 kbps. Also, the use of the power level of the reference pilot signal of 9.6 kbps that is the minimum data rate cannot provide the reference pilot signal level sufficient for the operation of 1024 kbps or 614.4 kbps.